The so-called narrow-neck press-and-blow method of manufacturing glass bottles is well known. Examples of this process are disclosed in Mumford, U.S. Pat. No. 3,216,813; Becker, U.S. Pat. No. 3,622,305; Becker, U.S. Pat. No. 3,644,111; Rowe, U.S. Pat. No. 3,765,862; and Foster, U.S. Pat. No. 4,009,016. Refinements have been made in the control of the process in an effort to make the blown bottles lighter in weight and more uniform in wall thickness.
The successful operation of the process in bottles having a 26 mm crown finish or 28 mm thread finish (which are the predominant beer and beverage sizes) depends on the maintenance of the volume of the blank cavity and the pressing plunger as well as the volume of the gob of glass to extremely exact specifications. Most bottle manufacturers do not attempt to operate the process on finishes below 38 mm in size because the cost of maintaining these close specifications exceeds the economic advantage of being able to reduce the weight of the glass in the bottle by the more uniform distribution of glass.
The need for this close specification control is created by the fact that the inside diameter of these finishes is only 3/4 of an inch, necessitating a pressing plunger diameter only slightly larger. The pressing plunger must be tapered in order to be withdrawn from the glass after the parison has been sufficiently chilled, because the glass shrinks around the plunger as it cools. The pressing plunger must also have a close fit with the guide ring in order to prevent the molten glass from squeezing into the joint between them and forming a rough seam on the finish of the bottle. Such a seam is highly objectionable for obvious reasons.
In this known press-and-blow process, whenever the volume of the hot glass exceeds the volume of the blank and finish cavity minus the volume of the plunger, an over-press results. If the volume of the hot glass is less than the volume of the blank and finish cavity minus the volume of the plunger, no pressing occurs because the fluid mass of glass is not confined. Obviously, the volume of the plunger in the blank and finish cavity varies by the amount of the penetration of the plunger. In the known process the variation in penetration is less than 1/4 of an inch, but it is continuously varying due to the variations in mold cavities and gob volumes. A normal set of blank cavities in this process consists of 24 to 40 individual cavities (including spares). The glass is supplied from two orifices supplying gobs at speeds of one hundred to one hundred thirty per minute from each orifice. The number of combinations of degree of variance is substantial.
With the continual variations in cavity capacity and glass gob volume the amount of penetration of the plunger into the blank mold varies from parison to parison. The result is a continual and substantial variation from one parison to another in the thickness of the glass between the top of the plunger and the surface of the baffle which seals the blank cavity. There is also a lesser but continual variation in the thickness of the side walls of the parison due to the taper of the plunger.
The primary object of the present invention is to produce glass bottle parisons in a narrow-neck press-and-blow operation by providing a process which is less demanding of maintenance of equipment and precision operations than present methods. The resulting parisons are more consistent in finish quality, wall thickness and bottom thickness and therefore capable of being expanded into bottles having improved glass distribution compared to present commercial production methods.
This objective is accomplished by using the mold cavity plunger only as a device to produce a bubble or cavity in the parison and limiting its travel to a predetermined position where it makes the best possible match with the guide ring (or the neck ring in the event that a guide ring is not used). The necessary pressure on the fluid mass of glass is provided separate from the cavity forming plunger. In one embodiment baffle means include a plunger means which is pressed against the fluid mass of the glass with the necessary force to produce the desired physical and thermal formation of the parison. In other embodiments a fluid is introduced into the mold and the fluid or gas pressure against the fluid mass provides the necessary force.
The providing of parisons having improved uniform size and shape and in which the glass distribution is consistent makes it possible to make bottles having improved glass distribution and strength and of lighter weight. Reduction in bottle weight is of primary importance since such reduction means a saving in energy cost, material cost, and transportation cost. Reducing the weight of a particular bottle enables faster cooling in the blow mold and often greater production per minute per mold cavity.
No one wishes to reduce the strength of the completed glass container. To measure the strength there are many tests such as vertical load tests, internal pressure tests and impact tests. On the basis of experience with test measurements, standards have been developed for minimum glass thickness allowable in the base, the heel, the sidewall, and the shoulder area of the glass bottle. There is also a minimum glass thickness for the wall of the finish which is required for satisfactory performance on filling machines and capping machines. The present invention makes it easier to produce blown containers by the press-and-blow process to meet all of the applicable customer standards with which I am familiar.
Much of the improved glass distribution in the finished bottle starts from improvement in the formation of the parison. Present practice produces a parison which is close to the height of the finished blow mold cavity so that blowing of the parison in the blow mold can start shortly after the delivery of the parison to the blow mold and the time required for the parison to sag in the blow mole is reduced. However, such long parisons are more difficult to control in the transfer from the blank mold to the blow mold and, further, exhibit variations in the glass distribution which are minimized by the present invention.